Sapphire thin film coated substrate

ABSTRACT

A method to transfer a layer of harder thin film substrate onto a softer, flexible substrate. In particular, the present invention provides a method to deposit a layer of sapphire thin film on to a softer and flexible substrate e.g. quartz, fused silica, silicon, glass, toughened glass, PET, polymers, plastics, paper and fabrics. This combination provides the hardness of sapphire thin film to softer flexible substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application of U.S.Non-provisional patent application Ser. No. 14/849,606 filed on Sep. 10,2015 which claims priority from U.S. provisional patent application No.62/049,364 filed on Sep. 12, 2014 and U.S. provisional patentapplication No. 62/183,182 filed on Jun. 22, 2015 and also is acontinuation-in-part application of: (1) U.S. Non-provisional patentapplication Ser. No. 14/642,742 filed on Mar. 9, 2015 which claimspriority from U.S. provisional patent application No. 62/049,364 filedon Sep. 12, 2014, (2) U.S. Non-provisional patent application Ser. No.13/726,127 filed on Dec. 23, 2012 which claims priority from U.S.provisional patent application No. 61/579,668 filed on Dec. 23, 2011,and (3) U.S. Non-provisional patent application Ser. No. 13/726,183filed on Dec. 23, 2012 which claims priority from U.S. provisionalpatent application No. 61/579,668 filed on Dec. 23, 2011; the presentapplication is also a continuation-in-part application of U.S.Non-provisional patent application Ser. No. 14/642,742 filed on Mar. 9,2015; the present application also claims priority from U.S. provisionalpatent application No. 62/339,074 filed on May 19, 2016, U.S.provisional patent application No. 62/375,433 filed on Aug. 15, 2016 andU.S. provisional patent application No. 62/405,215 filed on Oct. 6,2016; and the disclosures of which are incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to a method to transfer a layer of harderthin film substrate onto a softer substrate, especially onto a softer,flexible substrate. In particular, the present invention provides amethod to transfer a layer of sapphire thin film on to a softer flexiblesubstrate e.g. quartz, fused silica, silicon, glass, toughened glass,PET, polymer, plastic, paper, and fabric via a flip chip process. Thecombination of a layer of harder thin film sapphire substrate onto asofter substrate is better than pure sapphire substrate. In nature, theharder the materials, the more brittle they are. Thus, sapphiresubstrate is hard to scratch but it is easy to shatter, and the viceversa is also often true wherein quartz substrate is easier to bescratched but it is less brittle than sapphire substrate. Therefore,depositing a harder thin film substrate on a softer, flexible substratetakes the best of both worlds. Softer, flexible substrates are lessbrittle, have good mechanical performance and often cost less. Thefunction of anti-scratch is to be achieved by using the harder thin filmsubstrate.

BACKGROUND OF THE INVENTION

Sapphire is presently being actively considered as screen for smartphones and tablets. It is the second hardest material after diamond sousing it as screen would mean the smart phone/tablet has a superiorscratch and crack resistant screen. Sapphire screen is already beingfeatured in Apple iPhone 5S TouchID scanner and camera lens on the rearof the phone. Vertu, the luxury smartphone manufacturer, is alsodeveloping sapphire screen. However, since sapphire is the secondhardest material, it is also difficult to be cut and polished. Coupledby the fact that the growth of a single large size crystal sapphire istime consuming, this results in long fabrication time and highfabrication cost. It is the high fabrication cost and long fabricationtime of sapphire screen that limit Apple's use of such sapphire screento only Apple Watch.

A current popular ‘tough’ screen material use is the Gorilla Glass madeby Corning, which is being used in over 1.5 billion devices. Sapphire isin fact harder to be scratched than Gorilla Glass and this has beenverified by several third party institutes such as Center for AdvancedCeramic Technology at Alfred University's Kazuo Inamori School ofEngineering. On the Mohs scale of hardness, the newest Gorilla Glassonly scores 6.5 Mohs which is below the Mohs value of mineral quartz. Assuch, Gorilla Glass is still easy to be scratched by sand and metals.Sapphire is the second hardest naturally occurring material on theplanet, behind diamond which scores 10 on the Mohs scale of mineralhardness.

Mohs hardness test is to characterize the scratch resistance of mineralsthrough the ability of a harder material to scratch a softer material.It matches one substance's ability to scratch another, and so it is abetter indicator of scratch resistance than shatter resistance. This isshown in FIG. 1.

Following is quotations from ‘Display Review’ on sapphire screen:

“Chemically strengthened glass can be excellent, but sapphire is betterin terms of hardness, strength, and toughness” Hall explained, adding“the fracture toughness of sapphire should be around four times greaterthan Gorilla Glass—about 3 MPa-m0.5 versus 0.7 MPa-m0.5, respectively.”This comes with some rather large downsides though. Sapphire is bothheavier at 3.98 g per cubic cm (compared to the 2.54 g of Gorilla Glass)as well as refracting light slightly more.

So apart from being heavier, sapphire being the second hardest materialis also a difficult material to cut and polish. Growing single crystalsapphire is time consuming especially when the diameter size is large(>6 inches), this is technically very challenging. Therefore thefabrication cost is high and fabrication time is long for sapphirescreen. It is an objective of the present invention to providefabrication means of sapphire screen materials that is quick tofabricate and low in cost while having the following advantages:

-   -   Harder than any hardened glass;    -   Less possibility of fragmentation than pure sapphire screen;    -   Lighter weight than pure sapphire screen;    -   Higher transparency than pure sapphire screen.        For hardening of sapphire (Al₂O₃) thin film deposition,        softening/melting temperature of softer substrate should be        sufficiently higher than the annealing temperature. Most rigid        substrates such as quartz, fused silica can meet this        requirement. However, flexible substrate such as polyethylene        terephthalate (PET) would not be able to meet the requirement.        PET has a melting temperature of about 250° C., which is way        below the annealing temperature. PET is one of the most widely        used flexible substrates. The ability of transferring a        substrate of Al₂O₂ (sapphire) thin films on to a softer flexible        will significantly broaden its applications from rigid        substrates like glass and metals to flexible substrates like        PET, polymers, plastics, paper and even to fabrics. Mechanical        properties of transferred substrate can then be improved.        Therefore, Al₂O₃ thin films transfer from rigid substrate to        flexible substrate can circumnavigate this problem of the often        lower melting temperatures of flexible substrates.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a method to transfer a layer of harder thin film substrate ontoa softer, flexible substrate. In particular, the present inventionprovides a method to transfer a layer of sapphire thin film onto asofter, flexible substrate e.g. PET, polymers, plastics, paper and evento fabrics. This combination is better than pure sapphire substrate.

In accordance with a second aspect of the present invention, there isprovided a method for coating sapphire (Al₂O₃) onto a flexible substratecomprising: a first deposition process to deposit at least one firstthin film onto at least one first substrate to form at least one firstthin film coated substrate; a second deposition process to deposit atleast one second thin film onto the at least one first thin film coatedsubstrate to form at least one second thin film coated substrate; athird deposition process to deposit at least one catalyst onto the atleast one second thin film coated substrate to form at least onecatalyst coated substrate; a fourth deposition process to deposit atleast one sapphire (Al₂O₃) thin film onto the at least one catalystcoated substrate to form at least one sapphire (Al₂O₃) coated substrate;an annealing process wherein said at least one sapphire (Al₂O₃) coatedsubstrate annealed under an annealing temperature ranging from 300° C.to less than a melting point of sapphire (Al₂O₃) for an effectiveduration of time to form at least one hardened sapphire (Al₂O₃) thinfilm coated substrate; attaching at least one flexible substrate to theat least one hardened sapphire (Al₂O₃) thin film coated substrate on theat least one sapphire (Al₂O₃) thin film; a mechanical detachment processdetaching the at least one hardened sapphire (Al₂O₃) thin film togetherwith the at least one second thin film from the at least one first thinfilm coated substrate to form at least one second thin film coatedhardened sapphire (Al₂O₃) thin film on said at least one flexiblesubstrate; and an etching process removing the at least one second thinfilm from the at least one second thin film coated hardened sapphire(Al₂O₃) thin film on said at least one flexible substrate to form atleast one sapphire (Al₂O₃) thin film coated flexible substrate.

The method according to the second aspect of the present invention,wherein said first and/or said flexible substrate comprises at least onematerial with a Mohs value less than that of said at least one sapphire(Al₂O₃) thin film.

In a first embodiment of the second aspect of the present inventionthere is provided the method wherein said first and/or second and/orthird and/or fourth deposition process(es) comprise(s) e-beam depositionand/or sputtering deposition.

In a second embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one sapphire (Al₂O₃)coated substrate and/or at least one hardened sapphire (Al₂O₃) coatedsubstrate and/or at least one second thin film coated hardened sapphire(Al₂O₃) thin film on said at least one flexible substrate and/or atleast one sapphire (Al₂O₃) thin film coated flexible substratecomprise(s) at least one sapphire (Al₂O₃) thin film.

In a third embodiment of the second aspect of the present inventionthere is provided the method wherein a thickness of said at least onefirst substrate and/or said at least one flexible substrate is of one ormore orders of magnitude greater than the thickness of said at least onesapphire (Al₂O₃) thin film.

In a fourth embodiment of the second aspect of the present inventionthere is provided the method wherein the thickness of said at least onesapphire (Al₂O₃) thin film is about 1/1000 of the thickness of said atleast one first substrate and/or said at least one flexible substrate.

In a fifth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one sapphire (Al₂O₃)thin film has the thickness between 150 nm and 600 nm.

In a sixth embodiment of the second aspect of the present inventionthere is provided the method wherein said effective duration of time isno less than 30 minutes.

In a seventh embodiment of the second aspect of the present inventionthere is provided the method wherein said effective duration of time isno more than 2 hours.

In an eighth embodiment of the second aspect of the present inventionthere is provided the method wherein said annealing temperature rangesbetween 850° C. and 1300° C.

In a ninth embodiment of the second aspect of the present inventionthere is provided the method wherein said annealing temperature rangesbetween 1150° C. and 1300° C.

In a tenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one materialcomprising quartz, fused silica, silicon, glass, toughen glass, PET,polymers, plastics, paper, fabric, or any combination thereof; andwherein said material for the at least one flexible substrate is notetch-able by the at least one etching process.

In an eleventh embodiment of the second aspect of the present inventionthere is provided the method wherein said attachment between said atleast one flexible substrate and said at least one hardened sapphire(Al₂O₃) thin film is stronger than the bonding between said at least onefirst thin film and said second thin film.

In a twelfth embodiment of the second aspect of the present inventionthere is provided the method wherein the at least one first thin filmcomprises chromium (Cr) or any material that forms a weaker bond betweenthe at least one first thin film and the at least one second thin film;and wherein said material for the at least one first thin film is notetch-able by the at least one etching process.

In a thirteenth embodiment of the second aspect of the present inventionthere is provided the method wherein the at least one second thin filmcomprises silver (Ag) or any material that forms a weaker bond betweenthe at least one first thin film and the at least one second thin film;and wherein said material for the at least one second thin film isetch-able by the at least one etching process.

In a fourteenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one catalystcomprises a metal selected from a group consisting of titanium (Ti),chromium (Cr), nickel (Ni), silicon (Si), silver (Ag), gold (Au),germanium (Ge), and a metal with a higher melting point than that of theat least one first substrate.

In a fifteenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one catalyst coatedsubstrate comprises at least one catalyst film; wherein said at leastone catalyst film is not continuous; wherein said at least one catalystfilm has a thickness ranging between 1 nm and 15 nm; and wherein said atleast one catalyst film comprises a nano-dot with a diameter rangingbetween 5 nm and 20 nm.

In a third aspect of the present invention there is provided a methodfor coating sapphire on to a substrate comprising, an e-beam evaporationor sputtering deposition process at room temperature or 25° C., whereinsapphire is deposited directly on to a substrate selected from quartz,fused silica, silicon, glass, or toughened glass to form a sapphirecoated substrate, wherein the substrate during deposition is withoutexternal cooling or heating; and an annealing process, wherein saidsapphire coated substrate is annealed under an annealing temperatureranging between approximately room temperature or 25° C. and 2040° C.for an effective duration of time.

In a first embodiment of the third aspect of the present invention thereis provided the method for coating sapphire on to a substrate whereinsaid substrate comprises at least one material with a Mohs value lessthan that of said sapphire.

In a second embodiment of the third aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein said sapphire is deposited as a sapphire thin film on to saidsubstrate.

In a third embodiment of the third aspect of the present invention thereis provided the method for coating sapphire on to a substrate whereinsaid sapphire is deposited as a doped sapphire thin film on to saidsubstrate.

In a fourth embodiment of the third aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein the doped sapphire thin film is doped with doping elementcomprising one or more of chromium, chromium oxide, magnesium, magnesiumoxide, beryllium, beryllium oxide, lithium, lithium oxide, sodium,sodium oxide, potassium, potassium oxide, calcium, calcium oxide,molybdenum, molybdenum oxide, tungsten, and tungsten oxide.

In a fifth embodiment of the third aspect of the present invention thereis provided the method for coating sapphire on to a substrate whereinthe ratio of sapphire:doping element is in the range of 1:x, wherein xranges from 1 to 3.

In a sixth embodiment of the third aspect of the present invention thereis provided the method for coating sapphire on to a substrate wherein athickness of said substrate is of one or more orders of magnitudegreater than a thickness of said sapphire thin film.

In a seventh embodiment of the third aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein the thickness of said sapphire thin film is about 1/1000 of thethickness of said substrate.

In an eighth embodiment of the third aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein the thickness of said sapphire thin film is between 10 nm and1000 nm.

In a ninth embodiment of the third aspect of the present invention thereis provided the method for coating sapphire on to a substrate whereinsaid effective duration of time is no less than 30 minutes and no morethan 10 hours.

In a tenth embodiment of the third aspect of the present invention thereis provided a method for protecting a surface of a substrate by coatingsaid surface with sapphire using the method according to the presentinvention.

In an eleventh embodiment of the third aspect of the present inventionthere is provided a screen fabricated by using the method according tothe present invention for use in displays.

In a twelfth embodiment of the third aspect of the present inventionthere is provided a composition of sapphire coating made by the methodof the present invention used as a unique identifier of said sapphirecoating.

In a thirteenth embodiment of the third aspect of the present inventionthere is provided a sapphire-coated substrate made by the methodaccording to the present invention.

In a fourth aspect of the present invention there is provided a methodfor coating sapphire on to a substrate comprising a first e-beamevaporation or sputtering deposition process at room temperature or 25°C., wherein a buffer layer is deposited directly on to a substrateselected from polymers, plastics, paper, fabrics, PMMA, or PET to form abuffer coated substrate, wherein the substrate during deposition iswithout external cooling or heating; and a second e-beam evaporation orsputtering deposition process at room temperature or 25° C., whereinsapphire is deposited directly on to the buffer coated substrate to forma sapphire coated substrate, wherein the buffer coated substrate duringdeposition is without external cooling or heating; wherein the bufferlayer material has a mechanical hardness higher than that of thesubstrate and lower than that of the sapphire; and wherein the bufferlayer material has a refractive index higher than that of the substrateand lower than that of the sapphire.

In a first embodiment of the fourth aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein the mechanical hardness of said buffer layer material rangesfrom 1 to 5.5 Mohs scale.

In a second embodiment of the fourth aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein the reflective index of said buffer layer material ranges from1.45 to 1.65.

In a third embodiment of the fourth aspect of the present inventionthere is provided the method for coating sapphire on to a substratewherein said buffer layer material is comprising silicon dioxide andSiO₂.

In a fourth embodiment of the fourth aspect of the present inventionthere is provided a method for protecting a surface of a substrate bycoating said surface with sapphire using the method according to thepresent invention.

In a fourth embodiment of the fourth aspect of the present inventionthere is provided a screen fabricated by using the method according tothe present invention use in displays.

In a fifth embodiment of the fourth aspect of the present inventionthere is provided a sapphire-coated substrate made by the methodaccording to the present invention.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described.

The invention includes all such variation and modifications. Theinvention also includes all of the steps and features referred to orindicated in the specification, individually or collectively, and anyand all combinations or any two or more of the steps or features.

Other aspects and advantages of the invention will be apparent to thoseskilled in the art from a review of the ensuing description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the invention, whentaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows the Mohs scale of mineral hardness;

FIG. 2 shows the top-surface hardness of “Sapphire thin film on Quartz”when compared to ordinary glass, Gorilla Glass, quartz and puresapphire;

FIG. 3 shows the light transmittance of quartz, Sapphire thin film onQuartz and pure sapphire;

FIG. 4 shows the light transmission of quartz and 190 nm Sapphire thinfilm on Quartz with and without annealing at 1300° C. for 2 hours;

FIG. 5 shows XRD results for the 400 nm sapphire thin film on quartzannealed at 750° C., 850° C., and 1200° C. for 2 hours;

FIG. 6 shows the transmission spectrum of 400 nm sapphire thin film onquartz by e-beam with and without annealing at 1200° C. for 2 hourscomparing with quartz and sapphire substrates;

FIG. 7 shows the transmission spectrum of 160 nm sapphire thin film onfused silica by e-beam with and without annealing at 1150° C. for 2hours comparing with quartz and sapphire substrates;

FIG. 8A shows XRD results for the 400 nm sapphire thin film on quartzprepared by sputtering deposition and annealing at 850° C., 1050° C. and1200° C. for 2 hours;

FIG. 8B shows XRD results for the sapphire thin film with thicknesses of220 nm, 400 nm, and 470 nm on quartz prepared by sputtering depositionand annealing at 1150° C. for 2 hours;

FIG. 9 shows the transmission spectra of 220 nm, 400 nm and 470 nmsapphire thin film on quartz by sputtering deposition and annealing at1100° C. for 2 hours comparing with quartz substrate;

FIG. 10 shows XRD results for the 350 nm sapphire thin film on fusedsilica prepared by sputtering deposition and annealing at 750° C., 850°C., 1050° C., and 1150° C. for 2 hours;

FIG. 11 shows the transmission spectra of 180 nm-600 nm sapphire thinfilm on fused silica by sputtering deposition and annealing at 1150° C.for 2 hours comparing with fused silica substrate.

FIG. 12 shows the transmission of fused silica and 250 nm annealedsapphire thin film with or without 10 nm Ti catalyst on fused silicaannealing at 700° C. and 1150° C. for 2 hours;

FIG. 13A shows the X-ray reflectivity (XRR) measurement results fordifferent samples with different annealing conditions;

FIG. 13B shows the optical transmittance spectra for different sampleswith different annealing conditions;

FIG. 14 shows the EBL steps in the fabrication of the absorbermetamaterials with period of the disc-array device is 600 nm, discdiameter: 365 nm, thickness of gold: 50 nm, and thickness of Cr: 30 nm:FIG. 14A shows that the multilayer plasmonic or metamaterial device isfabricated on chromium (Cr) coated quartz; FIG. 14B shows that agold/ITO thin film is deposited onto the Cr surface; FIG. 14C shows thata ZEP520A (positive e-beam resist) thin film is spun on top of theITO/gold/Cr/quartz substrate and a two-dimensional hole array isobtained on the ZEP520A; FIG. 14D shows that a second gold thin film iscoated onto the e-beam patterned resist; and FIG. 14E shows that atwo-dimensional gold disc-array nanostructures is formed by removing theresist residue;

FIG. 14F shows the scanning electron microscope (SEM) image of the twodimensional gold disc-array absorber metamaterials;

FIG. 15 shows the schematic diagrams of the flip chip transfer method,the tri-layer absorber metamaterial with an area of 500 μm by 500 μm istransferred to a PET flexible substrate: FIG. 15A shows that adouble-sided sticky optically clear adhesive is attached to the PETsubstrate; FIG. 15B shows that a tri-layer metamaterial device accordingto an embodiment of the present invention is placed in intimate contactwith optical adhesive and sandwiched between the rigid substrate and theoptical adhesive; FIG. 15C shows that the Cr thin film on quartzsubstrate is exposed to the air for several hours after the RFsputtering process, such that there is a thin native oxide film on theCr surface; FIG. 15D shows that the tri-layer metamaterial nanostructureis peeled off from the Cr coated quartz substrate and transferred to aPET substrate; and FIG. 15E shows that the metamaterial nanostructure isencapsulated by spin-coating a PMMA layer on top of the device;

FIG. 16A and FIG. 16B show the flexible NIR absorber metamaterials on atransparent PET substrate; each separated pattern has an area size of500 μm by 500 μm;

FIG. 17 shows the relative reflection spectrum of the absorbermetamaterials on quartz substrate (gold disc/ITO/gold/Cr/quartz), NIRlight was normally focused on the device and the reflection signal andwas collected by the 15× objective lens, and blue line is theexperimental result and red line is the simulated reflection spectrumusing RCWA method;

FIG. 18 shows: FIG. 18A shows that Angle resolved back reflectionspectra measured on flexible metamaterial (with curved surface), thelight being incident from PET side and the back reflection was collectedby NIR detector; FIG. 18B shows that transmission spectra measured onthe flexible absorber metamaterial, the light being incident from thePMMA side was collected from the PET side; and FIG. 18C and FIG. 18D aresimulated reflection and transmission spectra, respectively, on flexibleabsorber metamaterial using RCWA method;

FIG. 19 shows experiment diagram of measuring the reflection spectrum ofmetamaterial device under different bending condition; the flexiblesubstrate was bent by adjusting the distance between A and B, and theincident angle 90°−φ (varying from 0 to 45 degree) was defined by theslope of PET substrate and direction of incident light;

FIG. 20 shows the fabrication structure for Al₂O₃ thin film transfer;

FIG. 21 shows the peeling off of Al₂O₃ thin film from the donorsubstrate;

FIG. 22 shows the etching of sacrificial Ag layer to complete the Al₂O₃thin film transfer to PET substrate;

FIG. 23 shows the fabrication sample of Al₂O₃ assembly ready for thinfilm transfer;

FIG. 24 shows the separation of Al₂O₃ from donor substrate;

FIG. 25 shows the nanoindentation results of aluminum oxide film on Sodalime glass (SLG) substrate with different post annealing conditions;

FIG. 26 shows the structure of the sample of a doped aluminum oxidelayer deposited on top of sapphire thin film;

FIG. 27 shows the nano-indentation measurement of different strengthenlayer with 300° C. annealing;

FIG. 28 shows the nano-indentation measurement of strengthen layer is1:1 (aluminum oxide:magnesium oxide) on SLG and ASS in room temperature;

FIG. 29 shows the transmittance of different strengthen layer with 300°C. annealing.

FIG. 30 shows the transmittance results of strengthen layer is 1:1(aluminum oxide:magnesium oxide) on SLG and ASS in room temperature;

FIG. 31 shows the GID of Al₂O₃:MgO at 1:1 on field silica (FS) atdifferent annealing temperatures.

FIG. 32 shows the average transmittance of selected PMMA samples withoutsapphire film, with sapphire film and with sapphire film in SiO₂; and

FIG. 33 shows the average hardness of selected PMMA samples withoutsapphire film, with sapphire film and with sapphire film in SiO₂.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is not to be limited in scope by any of thespecific embodiments described herein. The following embodiments arepresented for exemplification only.

Without wishing to be bound by theory, the present inventors havediscovered through their trials, experimentations and research that toaccomplish the task of transferring a layer of harder thin filmsubstrate onto a softer, flexible substrate e.g. PET, polymers,plastics, paper and even to fabrics. This combination is better thanpure sapphire substrate. In nature, the harder the materials, the morebrittle they are, thus, sapphire substrate is hard to scratch but it iseasy to shatter, and the vice versa is also often true wherein quartzsubstrate is easier to scratch but it is less brittle than sapphiresubstrate. Therefore, depositing a harder thin film substrate on asofter, flexible substrate gives the best of both worlds. Softer,flexible substrates are less brittle, have good mechanical performanceand cost less. The function of anti-scratch is to be achieved by usingthe harder thin film substrate. For hardening of sapphire (Al₂O₃) thinfilm deposition, softening/melting temperature of softer substrateshould be sufficiently higher than the annealing temperature. Most rigidsubstrates such as quartz, fused silica can meet this requirement.However, flexible substrate such as polyethylene terephthalate (PET)would not be able to meet the requirement. PET has a melting temperatureof about 250° C., which is way below the annealing temperature. PET isone of the most widely used flexible substrates. The ability oftransferring a substrate of Al₂O₃ (sapphire) thin films on to a softerflexible will significantly broaden its applications from rigidsubstrates like glass and metals to flexible substrates like PET,polymers, plastics, paper and even to fabrics. Mechanical properties oftransferred substrate can then be improved. Therefore, Al₂O₃ thin filmstransfer from rigid substrate to flexible substrate can circumnavigatethis problem of the often lower melting temperatures of flexiblesubstrates.

In accordance with a first aspect of the present invention, there isprovided a method to coat/deposit/transfer a layer of a harder thin filmsubstrate onto a softer substrate. In particular, the present inventionprovides a method to deposit a layer of sapphire thin film onto a softerflexible substrate e.g. PET, polymers, plastics, paper and fabrics. Thiscombination is better than pure sapphire substrate.

In accordance with a second aspect of the present invention, there isprovided a method for coating sapphire (Al₂O₃) onto flexible substratecomprising: a first deposition process to deposit at least one firstthin film onto at least one first substrate to form at least one firstthin film coated substrate; a second deposition process to deposit atleast one second thin film onto the at least one first thin film coatedsubstrate to form at least one second thin film coated substrate; athird deposition process to deposit at least one catalyst onto the atleast one second thin film coated substrate to form at least onecatalyst coated substrate; a fourth deposition process to deposit atleast one sapphire (Al₂O₃) thin film onto the at least one catalystcoated substrate to form at least one sapphire (Al₂O₃) coated substrate;an annealing process, wherein said at least one sapphire (Al₂O₃) coatedsubstrate is annealed under an annealing temperature ranging from 300°C. to less than a melting point of sapphire (Al₂O₃) for an effectiveduration of time to form at least one hardened sapphire (Al₂O₃) thinfilm coated substrate; attaching at least one flexible substrate to theat least one hardened sapphire (Al₂O₃) thin film coated substrate on theat least one sapphire (Al₂O₃) thin film; a mechanical detachment processdetaching the at least one hardened sapphire (Al₂O₃) thin film togetherwith the at least one second thin film from the at least one first thinfilm coated substrate to form at least one second thin film coatedhardened sapphire (Al₂O₃) thin film on said at least one flexiblesubstrate; and an etching process removing the at least one second thinfilm from the at least one second thin film coated hardened sapphire(Al₂O₃) thin film on said at least one flexible substrate to form atleast one sapphire (Al₂O₃) thin film coated flexible substrate.

The method according to the present invention, wherein said first and/orsaid flexible substrate comprises at least one material with a Mohsvalue less than that of said at least one sapphire (Al₂O₃) thin film.

In a first embodiment of the second aspect of the present inventionthere is provided the method wherein said first and/or second and/orthird and/or fourth deposition process comprise(s) e-beam depositionand/or sputtering deposition.

In a second embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one sapphire (Al₂O₃)coated substrate and/or at least one hardened sapphire (Al₂O₃) coatedsubstrate and/or at least one second thin film coated hardened sapphire(Al₂O₃) thin film on said at least one flexible substrate and/or atleast one sapphire (Al₂O₃) thin film coated flexible substratecomprise(s) at least one sapphire (Al₂O₃) thin film.

In a third embodiment of the second aspect of the present inventionthere is provided the method wherein a thickness of said at least onefirst substrate and/or said at least one flexible substrate is of one ormore orders of magnitude greater than the thickness of said at least onesapphire (Al₂O₃) thin film.

In a fourth embodiment of the second aspect of the present inventionthere is provided the method wherein the thickness of said at least onesapphire (Al₂O₃) thin film is about 1/1000 of the thickness of said atleast one first substrate and/or said at least one flexible substrate.

In a fifth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one sapphire (Al₂O₃)thin film has the thickness between 150 nm and 600 nm.

In a sixth embodiment of the second aspect of the present inventionthere is provided the method wherein said effective duration of time isno less than 30 minutes.

In a seventh embodiment of the second aspect of the present inventionthere is provided the method wherein said effective duration of time isno more than 2 hours.

In an eighth embodiment of the second aspect of the present inventionthere is provided the method wherein said annealing temperature rangesbetween 850° C. and 1300° C.

In a ninth embodiment of the second aspect of the present inventionthere is provided the method wherein said annealing temperature rangesbetween 1150° C. and 1300° C.

In a tenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one materialcomprising quartz, fused silica, silicon, glass, toughen glass, PET,polymers, plastics, paper, fabric, or any combination thereof; andwherein said material for the at least one flexible substrate is notetch-able by the at least one etching process.

In an eleventh embodiment of the second aspect of the present inventionthere is provided the method wherein said attachment between said atleast one flexible substrate and said at least one hardened sapphire(Al₂O₃) thin film is stronger than the bonding between said at least onefirst thin film and said second thin film.

In a twelfth embodiment of the second aspect of the present inventionthere is provided the method wherein the at least one first thin filmcomprises chromium (Cr) or any material that forms a weaker bond betweenthe at least one first thin film and the at least one second thin film;and wherein said material for the first thin film is not etch-able bythe at least one etching process.

In a thirteenth embodiment of the second aspect of the present inventionthere is provided the method wherein the at least one second thin filmcomprises silver (Ag) or any material that forms a weaker bond betweenthe at least one first thin film and the at least one second thin film;and wherein said material for the second thin film is etch-able by theat least one etching process.

In a fourteenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one catalystcomprises a metal selected from a group consisting of titanium (Ti),chromium (Cr), nickel (Ni), silicon (Si), silver (Ag), gold (Au),germanium (Ge), and a metal with a higher melting point than that of theat least one first substrate.

In a fifteenth embodiment of the second aspect of the present inventionthere is provided the method wherein said at least one catalyst coatedsubstrate comprises at least one catalyst film; wherein said at leastone catalyst film is not continuous; wherein said at least one catalystfilm has a thickness ranging between 1 nm and 15 nm; and wherein said atleast one catalyst film comprises a nano-dot with a diameter rangingbetween 5 nm and 20 nm.

Definitions

For clarity and completeness the following definition of terms used inthis disclosure:

The word “sapphire” when used herein refers to the material or substratethat is also known as a gemstone variety of the mineral corundumincluding those with different impurities in said material or substrate,an aluminium oxide (alpha-Al₂O₃), or alumina. Pure corundum (aluminumoxide) is colorless, or corundum with ˜0.01% titanium. The varioussapphire colors result from the presence of different chemicalimpurities or trace elements are:

-   -   Blue sapphire is typically colored by traces of iron and        titanium (only 0.01%).    -   The combination of iron and chromium produces yellow or orange        sapphire.    -   Chromium alone produces pink or red (ruby); at least 1% chromium        for deep red ruby.    -   Iron alone produces a weak yellow or green.    -   Violet or purple sapphire is colored by vanadium.

The word “harder” when used herein refers to a relative measure of thehardness of a material when compared to another. For clarity, when afirst material or substrate that is defined as harder than a secondmaterial or substrate, the Mohs value for the first material orsubstrate is higher than the Mohs value for the second material orsubstrate.

The word “softer” when used herein refers to a relative measure of thehardness of a material when compared to another. For clarity, when afirst material or substrate that is defined as softer than a secondmaterial or substrate, the Mohs value for the first material orsubstrate is lower than the Mohs value for the second material orsubstrate.

The word “flexible” when used herein refers to a substrate's mechanicalproperties of being able to be physically manipulated to change itsphysical shape using force without breaking said substrate.

The word “screen” when used as a noun herein refers to a cover-glass,cover-screen, cover-window, display screen, display window,cover-surface, or cover plate of an apparatus. For clarity, while inmany instances a screen on a given apparatus has a dual function ofdisplaying an interface of the apparatus and protecting the surface ofthe apparatus, wherein for such instances good light transmittance is arequired feature of said screen; this is not a must. In other instanceswhere only the function of providing surface protection is required,light transmittance of the screen is not a must.

In one embodiment of the present invention, there is provided a methodto develop a transparent screen which is harder and better than GorillaGlass and comparable to pure sapphire screen but with the followingadvantages:

-   -   Harder than any hardened glass;    -   Less possibility of fragmentation than pure sapphire screen;    -   Lighter weight than pure sapphire screen;    -   Higher transparency than pure sapphire screen.

In one embodiment of the present invention, there is provided a methodto deposit a sapphire thin film on quartz substrate. With post-deposittreatment such as thermal annealing, an embodiment of the presentinvention has achieved top-surface hardness up to 8-8.5 Mohs, which isclose to sapphire single crystal hardness of 9 Mohs. One embodiment ofthe present invention is herein known as “Sapphire thin film on Quartz”.FIG. 2 shows the top-surface hardness of “Sapphire thin film on Quartz”when compared to ordinary glass, Gorilla Glass, quartz and puresapphire.

Quartz substrate itself is the single crystal of SiO₂ with a higher Mohsvalue than glass. Moreover, its melting point is 1610° C. which canresist high annealing temperatures. Furthermore, the substrate can becut to the desired size onto which an embodiment of the presentinvention can then deposit the sapphire thin film. The thickness of thedeposited sapphire thin film is just 1/1000 of the quartz substrate. Thecost of synthetic quartz crystal is relatively low (which is only lessthan US $10/kg at the time the present invention is disclosed herein).So in an embodiment of the present invention, the fabrication cost andfabrication time are significantly reduced comparing to the fabricationof pure sapphire substrate.

Features and Benefits of One Embodiment of the Present Invention

Higher Hardness than Hardened Glass

In one embodiment of the present invention, the developed Sapphire thinfilm on Quartz has a maximum value of 8.5 Mohs in top-surface hardness.Recent Gorilla Glass used in smart-phone screen only scores about 6.5Mohs in hardness value and natural quartz substrate is 7 Mohs inhardness value. Therefore, the present invention has a significantimprovement in top-surface hardness comparing to recent technology. TheSapphire thin film on Quartz has a hardness value of 8.5 Mohs, which isvery close to pure sapphire's hardness value of 9 Mohs, and the Sapphirethin film on Quartz has the merits of lower fabrication cost andrequires a less fabrication time.

Less Fragmented, Lighter than Sapphire

In nature, the harder the materials, the more brittle they are, thus,sapphire substrate is hard to scratch but it is easy to shatter, and thevice versa is also often true. Quartz has comparatively low elasticmodulus, making it far more shock resistant than sapphire.

Moreover, in one embodiment of the present invention, the depositedsapphire thin film is very thin compared to quartz substrate wherein thedeposited sapphire thin film is only 1/1000 of the quartz substrate inthickness. Therefore, the overall weight of sapphire thin film on quartzis almost the same as quartz substrate, which is only 66.6% (or ⅔) ofthe weight of pure sapphire substrate for the same thickness. This isbecause the density of quartz is only 2.65 g/cm³ while that of puresapphire is 3.98 g/cm³ and that of Gorilla Glass is 2.54 g/cm³. In otherwords, quartz substrate is only heavier than Gorilla Glass by 4.3% butpure sapphire substrate is roughly 1.5 times heavier than Gorilla Glassand quartz. Table 1 shows the comparison among the density of quartz,Gorilla Glass and pure sapphire.

TABLE 1 Comparison of density of Gorilla glass, quartz and puresapphire, and their percentage differences. Materials Density DifferenceGorilla Glass 2.54 g/cm³  100% Quartz 2.65 g/cm³ 104.3% Pure Sapphire3.98 g/cm³ 156.7%

A recently published patent application, U.S. patent application Ser.No. 13/783,262 to Apple Inc., also indicates that it has devised a wayto fuse sapphire and glass layers together that creates a sapphirelaminated glass to combine the durability of sapphire with the weightand flexibility advantages of glass. However, polishing a larger area(>6 inches) and thin (<0.3 mm) sapphire substrate is very challenging.Therefore, using Sapphire thin film on Quartz is the best combinationfor screen with lighter weight, higher top-surface hardness, lessfragmented substrate.

Higher Transparency than Pure Sapphire

Since the refractive index of sapphire crystal, quartz crystal, andGorilla Glass are 1.76, 1.54, and 1.5 respectively, the overall lighttransmission of them are 85%, 91%, and 92% due to the Fresnel'sreflection loss. That means there is a small trade-off between lighttransmission and durability. Sapphire transmits less light which canresults in either dimmer devices or shorter device battery life. Whenmore light is transmitted, then more energy is saved and the devicebattery life would be longer. FIG. 3 shows the light transmittance ofquartz, Sapphire thin film on Quartz and pure sapphire.

Most crystals, including sapphire and quartz, have birefringenceproblem. By comparing their refractive indices of ordinary ray andextraordinary ray (n₀ and n_(e)), the magnitude of the difference Δn isquantified by the birefringence. Moreover, the values of Δn for oneembodiment of the present invention are also small such that thebirefringence problem is not serious for application with thinnersubstrate thickness (≤1 mm). For examples, pure sapphire is used as thecamera cover lens in Apple iPhone 5S, which is not known to have anyblurred image reported. Table 2 shows the refractive index of ordinaryray and extraordinary ray (n₀ and n_(e)), and their differences Δn inbirefringence for quartz and sapphire.

TABLE 2 Refractive indices of ordinary ray and extraordinary ray (n₀ andn_(e)), their differences Δn for quartz and sapphire. Materials Formulan₀ n_(e) Δn Quartz SiO₂ 1.544 1.553 +0.009 Sapphire Al₂O₃ 1.768 1.760−0.008

Shorter Fabrication Time and Lower Fabrication Cost than Pure Sapphire

Recently, both synthetic sapphire and quartz single crystals are grownand commercially available. Since sapphire has a higher melting pointthan quartz, the growth of sapphire is more difficult and in a highercost. More importantly, the time to grow sapphire is much longer thanquartz. Growing sapphire for products larger than 6 inches is alsochallenging and only a limited number of companies can achieve this.Therefore, it limits the production quantity such that production costof sapphire substrate is higher than quartz. Table 3 shows the chemicalformula, melting point and Mohs hardness value for quartz and sapphire.

TABLE 3 The chemical formula, melting point and Mohs hardness value forquartz and sapphire. Materials Formula Melting point Mohs hardnessQuartz SiO₂ 1610° C. 7 Sapphire Al₂O₃ 2040° C. 9

Another challenge in the use of pure sapphire is that sapphire crystalwith hardness value of 9 Mohs is very difficult to be cut and polished.Up to now, polishing a larger area (>6 inches) and thin (<0.3 mm)sapphire substrate is very challenging. The successful rate is not veryhigh and this prevents the price of sapphire substrate from anysignificant reduction even though a larger number of sapphire crystalgrowth furnaces are now in operation. Corning has claimed that sapphirescreen can cost up to 10 times as much as Gorilla Glass. In contrast,quartz possesses a hardness value of 7 Mohs, and it is easier to be cutand polished. Moreover, the cost of synthetic quartz crystal iscomparatively less expensive (only costs less than US $10/kg at the timeof the present disclosure).

Therefore, the additional cost of Sapphire thin film on Quartz is thedeposition of the sapphire thin film on the quartz substrate and thepost-treatment of the Sapphire thin film on Quartz. In one embodiment ofthe present invention, when all conditions are optimized, the process ofmass production can be fast and the cost is low.

In one embodiment of the present invention, there is provided a methodto deposit a harder sapphire thin film on quartz substrate. The thinfilm thickness is in the range of 150 nm-1000 nm. With post-deposittreatment such as thermal annealing at 500° C.-1300° C., this embodimentof the present invention has achieved hardness of 8-8.5 Mohs, which isvery close to sapphire single crystal hardness of 9 Mohs. In anotherembodiment of the present invention, there is provided sapphire thinfilm with thickness of 150 nm-500 nm with an achieved hardness value of8-8.5 Mohs, which is very close to sapphire single crystal hardness of 9Mohs, and also possesses good optical performance with low scatteringloss. The annealing temperature is from 1150 to 1300° C. FIG. 4 showsthe light transmission of quartz and 190 nm Sapphire thin film on Quartzwith and without annealing at 1300° C. for 2 hours. Therefore, in termsof hardness, the Sapphire thin film on Quartz is comparable to that ofpure sapphire screen, and its weight is almost the same as that ofglass/quartz substrate, which is roughly 66.6% the weight of puresapphire substrate since the density of quartz is only 2.65 g/cm³ whilepure sapphire is 3.98 g/cm³. Since one can cut the substrate to thedesired size then deposit the sapphire thin film according to thepresent method, the fabrication cost and time are significantly reducedcomparing to that of pure sapphire substrate.

In fact, the value of hardness for sapphire thin film by e-beamdeposition is not very high. In one embodiment of the present invention,the value of hardness was measured to be less than 7 Mohs. However,after conducting thermal annealing process, the thin film hardness issignificantly improved. In one embodiment of the present invention, itwas found that the sapphire thin film was softened as it was subjectedto annealing at 1300° C. for 2 hours. The film thickness was shrunkabout 10% and the film hardness was improved to 8-8.5 Mohs. Since, thequartz substrate is single crystal of SiO₂ with melting point of 1610°C., it can resist the high annealing temperature. Therefore, thehardness of the annealed sapphire thin film on quartz substrate canattain 8.5 Mohs. FIG. 4 shows the transmission of quartz and 190 nmthick Sapphire thin film on Quartz with and without annealing at 1300°C. for 2 hours.

Moreover, in other embodiments of the present invention, the annealingprocess of sapphire thin film can be conducted on other substrates. Forexamples, 1000° C. annealed sapphire thin film on fused silica substrateand 500° C. annealed sapphire thin film on glass substrate.

Electron beam (E-beam) and sputtering depositions are two most popularmethods to deposit sapphire thin film onto the quartz and other relevantsubstrates. In some embodiments of the present invention, these twocommon deposition methods are used.

Sapphire Thin Film by E-Beam Deposition

The summary points on sapphire thin film deposition on a given substrateby e-beam deposition are given as follows:

-   -   The deposition of sapphire thin film is using e-beam evaporation        since aluminum oxide has a very high melting point at 2040° C.        The white pellets or colorless crystal in small size of pure        aluminum oxide are used as the e-beam evaporating sources. The        high melting point of aluminum oxide also allows for annealing        temperatures up to less than the melting point of sapphire (e.g.        2040° C. at atmospheric pressure).    -   The substrates are perpendicularly stuck on the sample holder        far away from the evaporation source in 450 mm. The sample        holder is rotated at 1-2 RPM when the deposition takes place.    -   The base vacuum of evaporation chamber is less than 5×10−6 torr        and the vacuum keeps below 1×10−5 torr when the deposition takes        place.    -   The thickness of film deposited on substrates is about 150 nm to        1000 nm. The deposition rate is about 1-5 Å/s. The substrate        during deposition is without external cooling or heating. The        film thicknesses are measured by ellipsometry method and/or        scanning electron microscope (SEM).    -   Higher temperature film deposition is possible from room        temperature or 25° C. to 1000° C.

A more detailed description on the process of e-beam deposition forsapphire thin film on another substrate is given as follows:

-   -   1) The deposition of sapphire thin film is using e-beam        evaporation since aluminum oxide has a high melting point at        2040° C. The aluminum oxide pellets are used as the e-beam        evaporation source. The high melting point of aluminum oxide        also allows for annealing temperatures up to less than the        melting point of sapphire (e.g. 2040° C. at atmospheric        pressure).    -   2) The coated substrates are perpendicularly stuck on the sample        holder far away from the evaporation source in 450 mm. The        sample holder is rotated at 2 RPM when the deposition takes        place.    -   3) The thickness of film deposited on substrates is about 190 nm        to 1000 nm. The deposition rate is about 1 Å/s. The substrate        during deposition is without external cooling or heating. The        film thicknesses are measured by ellipsometry method.    -   4) After deposition of sapphire thin film on substrates, they        are annealed in a furnace from 500° C. to 1300° C. The        temperature raising speed is 5° C./min and the decline speed is        1° C./min. The time ranges from 30 minutes to 2 hours, keeping        on the particular thermal annealing temperature.    -   5) The deposition substrates are including quartz, fused silica        and (toughened) glass. Their melting points are 1610° C.,        1140° C. and 550° C. respectively. The annealing temperatures of        sapphire thin film coated on them are 1300° C., 1000° C. and        500° C. respectively.    -   6) The transmission of quartz and 190 nm sapphire thin film on        quartz with and without annealing at 1300° C. for 2 hours are        showed in FIG. 4. The light transmission percentage in whole        visible region from 400 nm-700 nm is greater than 86.7% and        maximally 91.5% at 550 nm while for pure sapphire substrate the        light transmission percentage is only 85-86%. More light        transmitted indicates more energy saved from backlight-source of        display panel, so such that the device battery life would be        longer.

Annealing Process of an Embodiment of the Present Invention

After deposition of sapphire thin film on substrates, they are annealedin a furnace from 500° C. to 1300° C. The temperature raising rate is 5°C./min and the decline rate is 1° C./min. The annealing time is from 30minutes to 2 hours, maintaining at a particular thermal annealingtemperature. Multiple-steps annealing with different temperatures withinthe aforementioned range are also used to enhance the hardness and alsoreduce the micro-crack of thin film. Table 4 shows the surface hardnessand XRD characteristic peaks at different annealing temperaturesprepared by e-beam deposition. The table also shows various crystallinephases of sapphire present in the films; most common phases are alpha(α), theta (θ), and delta (δ).

TABLE 4 The surface hardness and XRD characteristic peaks at differentannealing temperatures prepared by e-beam deposition. Annealingtemperature Surface hardness XRD peaks (° C.) (Mohs) (phase) Noannealing 5.5 No 500-850 6-7 No  850-1150 7-8 theta & delta 1150-1300 8-8.5 theta & delta

Table 4 shows the changes of surface hardness of sapphire thin film as afunction of annealing temperature varies from 500° C. to 1300° C. Infact, the initial value of hardness of e-beam deposited sapphire thinfilm without being annealed is about 5.5 Mohs. However, after conductingthermal annealing process, the film hardness is significantly improved.By using annealing temperature in the ranges of 500° C.-850° C., 850°C.-1150° C., and 1150° C.-1300° C., the hardness values of sapphire thinfilm on quartz are 6-7 Mohs, 7-8 Mohs and 8-8.5 Mohs in hardness scalerespectively.

FIG. 5 shows XRD results for the 400 nm sapphire thin film on quartzannealed at 750° C., 850° C. and 1200° C. for 2 hours. When theannealing temperature is greater than 850° C., the film starts topartially crystallize. The appearance of new XRD peaks corresponds tothe mixture of theta and delta structural phases of aluminum oxide.

When the annealing temperature is above 1300° C., the film would startto develop some larger crystallites that can significantly scattervisible light; this would reduce the transmission intensity. Moreover,as this large crystallite accumulates more and more, the film wouldcrack and some micro-size pieces would detach from the substrate.

In one embodiment of the present invention, it was found that thesapphire thin film on quartz substrate can be annealed at 1150° C. to1300° C. within half to two hours. The film thickness would shrink byabout 10% and the film hardness is improved to 8-8.5 Mohs. Since thequartz substrate is single crystal SiO₂ with a melting point of 1610°C., it can resist such high annealing temperature. Under this annealingtemperature, the hardness of annealed sapphire thin film on quartzsubstrate has achieved 8.5 Mohs.

The light transmission of 400 nm Sapphire thin film on Quartz with andwithout annealing at 1200° C. for 2 hours are shown in FIG. 6 whilecomparing to quartz and sapphire substrates. The light transmission ofSapphire thin film on Quartz within visible region, from 400-700 nm, isgreater than 88% and the maximum is at 550 nm with 92%. The interferencepattern is due to the differences in refractive index of the materialsand the film thickness. The overall averaging light transmittance isabout 90% while pure sapphire substrate is only 85-86%. Moreover, thelight transmission spectrum of Sapphire thin film on Quartz coincideswith that of quartz substrate at certain wavelength which indicates theoptical performance is excellent and low scattering loss. The differencebetween maximum and minimum intensity of the interference pattern isabout 4% only. For real applications, more light transmitted indicatesmore energy saved from backlight-source of display panel, so such thatthe device battery life would be longer.

Thickness of Sapphire Thin Film on Quartz

The Sapphire thin film on Quartz with thickness in the range of 150nm-1000 nm has been tested. In one embodiment of the present invention,there is provided a sapphire thin film with a thickness of 150 nm-500 nmhaving good optical performance with low scattering loss when annealingtemperature is from 1150° C. to 1300° C. However when the thickness islarger than 600 nm, the film would crack causing significant scatteringwhich reduces the transmission intensity.

For the sapphire thin film with thickness of 150 nm-500 nm deposited onquartz after annealing at 1150° C. to 1300° C., the measured hardnesscan achieve 8-8.5 in Mohs scale, which indicates that even thinnercoating film can also act as an anti-scratching layer.

Other Possible Substrates for Anti-Scratch Coating

Apart from quartz substrate, other embodiments of the present inventionhave also investigated the deposition of sapphire thin film on differentsubstrates such as fused silica and silicon. Other tempered glass ortransparent ceramic substrates with a higher annealing or meltingtemperature, which can resist 850° C. annealing temperature within 30minutes to 2 hours, are also possible to use as substrates to enhancetheir surface hardness to 7-8 in Mohs hardness scale. For example,Schott Nextrema transparent ceramics has a short heating temperature at925° C.; Corning Gorilla glass has a softening temperature up to 850° C.

Since the annealing temperature of fused silica is about 1160° C., it isa good candidate to start investigating its suitability as substrate.However, sapphire thin film on fused silica shows different behaviorscompared with sapphire thin film on quartz annealing from 850° C. to1150° C., even though they are deposited under the same depositioncondition. The adhesion of sapphire film on fused silica is not as goodas on quartz (due to significant difference in the expansioncoefficient); localized delamination and micro-sized crack of the filmoccur on fused silica substrate. However, using thinner film, theseproblems, which can lead to light scattering, are substantiallymitigated. FIG. 7 showed the transmission of 160 nm sapphire thin filmon fused silica annealed at 1150° C. for 2 hours. The transmission ofsapphire thin film on fused silica in whole visible region from 400nm-700 nm is greater than 88.5% and maximally 91.5% at 470 nm. Theoverall averaging light transmittance percentage is about 90% while puresapphire substrate is only 85%-86%. Moreover, the measured surfacehardness also maintains at above 8 in Mohs scale.

Silicon, which has a melting temperature at about 1410° C., is anon-transparent substrate material. Under the same deposition condition,although sapphire film on silicon substrate shows similarcharacteristics in Mohs hardness comparing to quartz substrate, they arestill divided into the two groups of temperature range. However, becausesilicon substrate is not a transparent substrate, it cannot be used intransparent cover glass or window applications. Therefore, the sapphirefilm can only provide the anti-scratch purpose as a protection layer toprotect the silicon surface from scratch (silicon has Mohs scalehardness of 7). Such protection layer can potentially eliminate thickglass encapsulation. This would improve the light absorption, thusincreasing the light harvesting efficiency. Other inorganicsemiconductor-based solar cell that can withstand high temperaturetreatment can also have similar deposition of the sapphire thin filmonto it. From the embodiments of the present invention as describedherein, it is envisaged that a person skilled in the art can very wellapply the present invention to deposit sapphire thin film on to othersubstrates such that the sapphire thin film will act as an anti-scratchprotection layer to its underlying substrate provided these substratescan withstand the annealing temperatures of the present invention forthe applicable duration of time.

Annealed Sapphire Thin Film by Sputtering Deposition

Sapphire Thin Film by Sputtering Deposition

The steps on sapphire thin film deposition on a given substrate bysputtering deposition are provided as follows:

-   -   1) The deposition of sapphire thin film can be performed by        sputtering deposition using aluminum or aluminum oxide targets.    -   2) The substrates are attached onto the sample holder which is        around 95 mm away from the target. The sample holder is rotated        to achieve thickness uniformity when the deposition takes place,        example rate is 10 RPM.    -   3) The base vacuum of evaporation chamber is less than 3×10⁻⁶        mbar and the coating pressure is around 3×10−3 mbar.    -   4) The thickness of film deposited on substrates is about 150 nm        to 600 nm.    -   5) Higher temperature film deposition is possible from room        temperature or 25° C. to 500° C.

Annealing Process of Another Embodiment of the Present Invention

After deposition of sapphire thin film on substrates, they are annealedin a furnace under a varying temperature from 500° C. to 1300° C. Thetemperature raising rate is 5° C./min and the decline rate is 1° C./min.The time ranges from 30 minutes to 2 hours, maintaining at a particularthermal annealing temperature. Multiple-step annealing at differenttemperatures are also used to enhance the hardness and also reduce themicro-crack of thin film. This is shown in Table 5.

TABLE 5 The surface hardness and XRD characteristic peaks at differentannealing temperatures for the sapphire film on quartz prepared bysputtering deposition. Annealing Surface Temperature Thickness hardnessXRD peaks (° C.) (nm) (Mohs) (phase) Transmission No annealing 6-6.5 No500-850 6-6.5 No  850-1150 340-600 Film theta & delta delamination1150-1300 150-300 8-8.5 theta & delta Low scattering 90% 300-5008.5-8.8  alpha & theta; High scattering alpha only 83-87%

Table 5 shows the changes of surface hardness of sapphire thin film onquartz as annealing temperature varies from 500° C. to 1300° C. In fact,the initial value of hardness of sapphire thin film without annealing bysputtering deposition is slightly higher than that by e-beam deposition;about 6-6.5 Mohs. After conducting thermal annealing process, theperformance of the film in terms of hardness is different from that bye-beam deposition. When annealing temperature is in the range of 500°C.-850° C., the film hardness has no significant change. Within 850°C.-1150° C. range, the thin film coated on quartz is easily delaminated.However, within 1150° C.-1300° C. range, the film forms hard film, withits surface hardness of 8-8.5 Mohs in a thickness of 150 nm-300 nm andof 8.5-8.8 Mohs in a thickness of 300 nm-500 nm.

FIG. 8A shows XRD results for the 400 nm sapphire thin films on quartzbeing annealed at 850° C., 1050° C. and 1200° C. for 2 hours. Theoccurring XRD peaks are corresponding to the mixing of delta (δ), theta(θ) and alpha (α) structural phases of aluminum oxide. Different frome-beam evaporation, the occurrence of alpha phase of aluminum oxide inXRD result by sputtering deposition causes more hardened surface orhigher surface hardness, scoring 8.7 Mohs in average. FIG. 8B shows XRDresults for the sapphire thin film with thicknesses of 220 nm, 400 nm,and 470 nm on quartz being annealed at 1150° C. for 2 hours. Theoccurrence of alpha phase starts from the thickness of about 300 nm, andwhen the thickness of sapphire thin film increases up to 470 nm, theoriginal mixing of structural phases almost converts to alpha phase. Thesurface hardness is the highest under such conditions. However, furtherincreasing the thickness of sapphire thin film would cause filmdelamination.

The light transmission spectra of 220 nm, 400 nm, and 470 nm sapphirethin film on quartz prepared by sputtering deposition being annealed at1100° C. for 2 hours are shown in FIG. 9 while comparing to quartzsubstrate. For annealed 220 nm thick sapphire thin film on quartz, theoptical performance is excellent and with a little scattering loss. Thetransmission in whole visible region from 400 nm-700 nm is greater than87% and maximally 91.5% at 520 nm. The overall averaging transmittanceis about 90.2%. The difference between the maximum and minimumintensities of the interference pattern is about 4.5% only.

However, when the thickness of sapphire thin film is greater than 300nm, the light transmittance intensity starts to drop, especially in UVrange, indicating that Rayleigh scattering starts to dominate. Thestrong wavelength dependence of Rayleigh scattering applies to thescattering particle with particle size, which is less than 1/10wavelength. This is due to the formation of alpha phase in sapphire thinfilm with sub-100 nm crystalline size. Therefore, the surface hardnessbecomes higher but the transmission becomes worse.

For annealed 400 nm and 470 nm sapphire thin film on quartz, the lighttransmission percentage in whole visible region from 400 nm-700 nm iswithin 81%-88% and 78%-87% respectively. Their overall averagingtransmittance values are about 85.7% and 83.0% respectively.

However, when the thickness of sapphire thin film is greater than 500nm, larger crystallite accumulates with micro-cracks form, the filmwould crack and some micro-size pieces would detach from the substrate.

Sapphire Thin Film on Fused Silica by Sputtering Deposition

Apart from quartz substrate, low cost fused silica is a potentialcandidate for sapphire thin film coated substrates since the annealingtemperature of fused silica is about 1160° C.

Table 6 showed the surface hardness of sapphire thin film on fusedsilica as annealing temperature varies from 750° C. to 1150° C. In fact,the initial value of hardness of sapphire thin film on fused silicawithout annealing by sputtering deposition is slightly lower than thaton quartz; about 5.5-6 Mohs. For 850° C.-1150° C. range, the hardness iseven worse, less than 5 Mohs for all 150 nm-600 nm thick sapphire thinfilms. However, at 1150° C., the film can form hard film again, whichits surface hardness has 8-8.5 for all 150 nm-600 nm sapphire thinfilms.

TABLE 6 The surface hardness and XRD characteristic peaks at differentannealing temperatures for the sapphire film on fused silica prepared bysputtering deposition. Annealing Surface Temperature Thickness hardnessXRD peaks (° C.) (nm) (Mohs) (phase) Transmission No annealing 5.5-6   No  850-1150 150-600 <5 theta & delta 1150-1300 150-300 8-8.5 theta &delta Low scattering 91% 300-600 8-8.5 alpha & theta; High scatteringalpha only 74-82%

FIG. 10 shows XRD results for the 350 nm thick sapphire thin film onfused silica prepared by sputtering deposition and annealing at 750° C.,850° C., 1050° C. and 1150° C. for 2 hours. XRD results show the mixingof theta and alpha structural phases of aluminum oxide co-exist on thefused silica substrate. Therefore, the sapphire thin film has a hardsurface with 8-8.5 Mohs, whereas fused silica substrate has only scores5.3-6.5.

The transmission spectra of 180 nm-600 nm thick sapphire thin film onfused silica prepared by sputtering deposition annealing at 1150° C.with 2 hours showed in FIG. 11 compared to fused silica substrate.

For annealed 180 nm and 250 nm thick sapphire thin film on fused silica,the optical performance is excellent and with a little scattering loss.The transmission of sapphire thin film in whole visible region from400-700 nm is within 88.9%-93.1% and 84.8%-92.8% respectively. Theiroverall averaging transmittance values are about 91.3% and 90.7%respectively.

For annealed 340 nm and 600 nm thick sapphire thin film on fused silica,the transmission across visible region from 400 nm-700 nm is within75%-86% and 64%-80% respectively. Their overall averaging transmittanceis about 81.7% and 74.1% respectively.

Therefore, annealed sapphire thin film on fused silica at 1150° C. witha thickness of 150 nm-300 nm has good optical performance with about 91%transmittance and also has strong surface hardness with >8 Mohs.

Low Temperature Annealing Process

A current popular ‘toughened’ screen material is Gorilla Glass fromCorning, which is being used in over 1.5 billion devices. On the Mohsscale of hardness, the latest Gorilla Glass only scores 6.5-6.8, whichis below mineral quartz such that it is still easy to scratch by sand.Therefore, another approach is to deposit harder thin film on glasssubstrate. However, for most of common cover glasses, the allowedmaximum annealing temperatures are in the range of 600° C.-700° C. Atthis temperature range, the previous hardness of annealed sapphire thinfilm can only reach 6-7 Mohs, which is close to that of glass substrateitself. Therefore, a new technology is developed to push the Mohshardness of annealed sapphire thin film to over 7 using annealingtemperature below 700° C.

In another embodiment of the present invention, a layer or multilayer ofhigher hardness thin film of sapphire is deposited onto a weakerhardness substrate (e.g. Gorilla glass, toughened glass, soda-limeglass, etc.) with maximum allowed annealing temperature below 850° C.Therefore, a harder anti-scratch thin film can be coated onto glass.This is the quickest lower cost way to improve their surface hardness.

In yet another embodiment of the present invention, by applying anano-layer of metal, such as Ti and Ag, it is shown that polycrystallinesapphire thin film can be grown at lower temperature. This catalyticenhancement can be induced at temperature considerably lower than whenthe nano-metal catalyst is not used. The enhancement comes from enablingcrystallization established once there is sufficient kinetic energy toallow deposited atoms to aggregate and this annealing temperature canstart at 300° C. Embodiments of the present invention wherein the lowtemperature annealing starting from 300° C. is presented in Table 7.

TABLE 7 Embodiments with structure of Substrate/Ti catalyst/Sapphirefilm with no annealing (Room Temperature, i.e. RT, or 25° C.), annealingtemperatures of 300° C., 400° C., and 500° C. Sapphire Knoop IncrementSubstrate Annealing Annealing Ti catalyst film hardness in Knoop typetemperature time thickness thickness (HK0.01) hardness Fused silica RT // / 1100 / Fused silica 300° C. 2 hrs 1.5 nm 250 nm 1101 +0.09% Fusedsilica 400° C. 2 hrs 1.5 nm 250 nm 1250 +13.64% Fused silica 500° C. 2hrs 1.5 nm 250 nm 1301 +18.27% Fused silica 300° C. 2 hrs 3.0 nm 250 nm1182 +7.45% Fused silica 400° C. 2 hrs 3.0 nm 250 nm 1276 +16.00% Fusedsilica 500° C. 2 hrs 3.0 nm 250 nm 1278 +16.18% Soda lime RT / / / 788 /glass Soda lime 300° C. 2 hrs 7.5 nm 230 nm 904 +14.72% glass Soda lime400° C. 2 hrs 7.5 nm 230 nm 977 +23.98% glass Soda lime 500° C. 2 hrs7.5 nm 230 nm 1052 +33.50% glass

FIG. 13A shows the X-ray reflectivity (XRR) measurement results fordifferent samples with different annealing conditions as per embodimentin Table 7, while FIG. 13B shows the optical transmittance spectra fordifferent samples with different annealing conditions as per embodimentin Table 7.

In one embodiment, a method is developed to deposit a very thin‘discontinuous’ metal catalyst and a thicker sapphire film on glasssubstrate. With post-deposit treatment such as thermal annealing at600-700° C., hardness of 7-7.5 Mohs is achieved, which is higher thanthat of most glasses.

The nano-metal catalyst should have a thickness between 1-15 nmdeposited by deposition system such as e-beam evaporation or sputtering.This catalyst is not a continuous film, as shown by SEM. The depositedmetal can have a nano-dot (ND) shape with (5-20 nm) diameter. The metalsinclude Titanium (Ti), and silver (Ag). The thicker sapphire film is inthe range of 100-1000 nm.

In fact, the hardness value of sapphire thin film by e-beam orsputtering deposition is not very high, which is about 5.5-6 Mohs only.However, after thermal annealing process, the film hardness issignificantly improved. Without nano-metal catalyst, the film hardnessis about 6-7 Mohs with annealing temperature 600-850° C. After addingthe nano-metal catalyst, the film hardness is improved to 7-7.5 Mohswith annealing temperature of 600-700° C. and achieved with a hardnessof 8.5 to 9 Mohs with annealing temperature of 701-1300° C.

This is a great improvement of surface hardness on glass substrate andin particular it is below the glass softening temperature at thisannealing temperature. This means that glass will not deform during theannealing. Thus, the role of metal catalyst not only enhances theadhesion between sapphire thin film and glass substrate but also inducesthe hardening of the sapphire thin film. The surface hardness ofsapphire thin film with and without nano-metal catalyst at differentannealing temperature ranges prepared by e-beam deposition is shown inTable 8.

TABLE 8 The surface hardness of sapphire thin film with and withoutnano-metal catalyst at different annealing ranges prepared by e-beamdeposition. Annealing Surface hardness without Surface hardness withtemperature nano-metal catalyst nano-metal catalyst (° C.) (Mohs) (Mohs)No annealing 5.5 5.5-6  500/600-850 6-7  7-7.5  850-1150 7-8 7.5-8.51150-1300  8-8.5 8.5-8.8

The summary points on sapphire thin film deposited on a glass substrateby e-beam deposition are given as follows:

1) The base vacuum of evaporation chamber is less than 5×10⁻⁶ torr andthe deposited vacuum keeps below 1×10⁻⁵ torr when the deposition takesplace.

2) The substrates are attached onto the sample holder at a distance fromthe evaporation source, for example 450 mm. The sample holder is rotatedat 1-2 RPM when the deposition takes place.

3) The deposition of nano-metals with higher melting points such as Ti,Cr, Ni, Si, Ag, Au, Ge and etc., is using deposition system such ase-beam evaporation and sputtering. The thickness of metal catalystdirectly deposited on substrates is about 1-15 nm monitoring by QCMsensor. The deposition rate of nano-metal catalyst is about 0.1 Å/s. Thesubstrate during deposition is without external cooling or heating. Thefilm morphology was measured by SEM top-view and cross-section view.

4) The deposition of sapphire thin film is using e-beam evaporationsince it has very high melting point at 2040° C. The white pellets orcolorless crystal in small size of pure aluminum oxide are used as thee-beam evaporating sources. The high melting point of aluminum oxidealso allows for annealing temperatures up to less than the melting pointof sapphire (e.g. 2040° C. at atmospheric pressure).

5) The thickness of sapphire thin film deposited on substrates is about100 nm to 1000 nm. The deposition rate is about 1-5 Å/s. The substrateduring deposition is at room temperature or 25° C. and activetemperature is not essential. The film thicknesses can be measured byellipsometry method or other appropriate methods with similar or betteraccuracy.

6) After deposition of sapphire thin film on substrates, they areannealed in a furnace with a temperature varying from 500° C. to 1300°C. The temperature raising gradient should be gradual for example 5°C./min and the decline gradient should also be gradual for example 1-5°C./min. The annealing time ranges from 30 minutes to 10 hours within thespecified thermal annealing temperature range. Multiple-steps annealingwith different temperatures within the aforementioned range can also beused to enhance the hardness and also reduce the micro-crack of thinfilm.

The transmission of fused silica and 250 nm annealed sapphire thin filmwith or without 10 nm Ti catalyst on fused silica annealing at 700° C.and 1150° C. for 2 hours are shown in FIG. 12. For 700° C. annealingresult, the averaged transmission percentage in visible region from400-700 nm is greater than 89.5% and reaches a maximum of 93.5% at 462nm while fused silica substrate has an average transmission of 93.5%.

Thin Film Transfer Process

Another embodiment of present invention provides a method and apparatusof fabrication of a multilayer flexible metamaterial using flip chiptransfer (FCT) technique. Such metamaterial includes a thin film hardersubstrate transferred onto a softer flexible substrate. This techniqueis different from other similar techniques such as metal lift offprocess, which fabricates the nanostructures directly onto the flexiblesubstrate or nanometer printing technique. It is a solution-free FCTtechnique using double-side optical adhesive as the intermediatetransfer layer and a tri-layer metamaterial nanostructures on a rigidsubstrate can be transferred onto adhesive first. Another embodiment ofthe present invention is the fabrication method and apparatus thatallows the transfer of the metamaterial from a rigid substrate such asglass, quartz and metals onto a flexible substrate such as plastic orpolymer film. Thus, a flexible metamaterial can be fabricatedindependent of the original substrate used.

Device Fabrication

A schematic fabrication process of multilayer metamaterials is shown inFIG. 14. First, the multilayer plasmonic or metamaterial device isfabricated on chromium (Cr) coated quartz using conventional EBLprocess. The 30 nm thick Cr layer is used as a sacrificial layer. Then agold/ITO (50 nm/50 nm) thin film is deposited onto the Cr surface usingthermal evaporation and RF sputtering method respectively. Next, aZEP520A (positive e-beam resist) thin film with a thickness of about 300nm is spun on top of the ITO/gold/Cr/quartz substrate and atwo-dimensional hole array is obtained on the ZEP52 OA using the EBLprocess. To obtain the gold nanostructure (disc pattern), a second 50 nmthick gold thin film is coated onto the e-beam patterned resist.Finally, a two-dimensional gold disc-array nanostructures is formed byremoving the resist residue. The area size of each metamaterial patternis 500 μm by 500 μm, and the period of the disc-array is 600 nm withdisc diameter of ˜365 nm.

Flip Chip Transfer (FCT) Technique

Transfer process of flexible absorber metamaterial is shown in FIG. 15,double-sided sticky optically clear adhesive (50 μm thick; e.g. acommercially available product manufactured by 3M) is attached to thePET substrate (70 μm thick). Thus, the tri-layer metamaterial device isplaced in intimate contact with optical adhesive and sandwiched betweenthe rigid substrate and the optical adhesive. Note that the Cr thin filmon quartz substrate is exposed to the air for several hours after the RFsputtering process, such that there is a thin native oxide film on theCr surface. Hence the surface adhesion between Cr and gold is muchweaker than that of gold/ITO/gold disc/optical adhesive bounding. Thisallows the tri-layer metamaterial nanostructure to be peeled off fromthe Cr coated quartz substrate. Once the metamaterial nanostructure istransferred onto the PET substrate, it possesses sufficient flexibilityto be bended into various shapes. Finally, the metamaterialnanostructure is encapsulated by spin-coating a 300 nm thick PMMA layeron top of the device.

In another embodiment, the present invention provides a novel NIRmetamaterial device that can be transformed into various shapes bybending the PET substrate.

FIG. 16A shows the flexible absorber metamaterial sandwiched by thetransparent PET and PMMA thin film. Several absorber metamaterialnanostructures with area size of 500 μm by 500 μm are fabricated onflexible substrate. In fact, using the flexibility property of the PETlayer, the absorber metamaterial device can be conformed into many shapee.g. cylindrical shape (FIG. 16B). The minimum radius of the cylindricalsubstrate is about 3 mm, not obvious defect on the metamaterial devicecan be observed after 10 times of repeatable bending tests.

Optical Characterization and Simulation

The tri-layer metal/dielectric nanostructure discussed above is anabsorber metamaterial device. The design of the device is such that theenergy of incident light is strongly localized in ITO layer. Theabsorbing effects of the NIR tri-layer metamaterial architecture couldbe interpreted as localized surface plasmin resonance or magneticresonance. The absorbing phenomenon discussed here is different from thesuppression of transmission effect in metal disc arrays, in which theincident light is strongly absorbed due to resonance anomaly of theultra thin metal nanostructure. To characterize the optical property ofgold disc/ITO/gold absorber metamaterial, Fourier transform infraredspectrometer (FTIR) is used to measure the reflection spectrum of theabsorber metamaterial. By combining the infrared microscope with theFTIR spectrometer, transmission and reflection spectra from micro-areananophotonic device can be measured. In FIG. 17, the reflection spectrum(Experiment line plot) from air/metamaterial interface was measured withsampling area of 100 μm by 100 μm. At the absorption peak withwavelength of ˜1690 nm, reflection efficiency is about 14%, i.e. theabsorber metamaterial works at this wavelength. In RCWA simulation(Simulation line plot), the real optical constants in E. D. Palik,Handbook of optical constants of solids, Academic Press, New York, 1985is used; the content of which is incorporated herein by reference in itsentirety. At resonant wavelength, the experiment and calculation agreewell with each other.

Reflection spectrum of the flexible absorber metamaterial is shown inFIG. 18A (0° line plot). Compared to the FTIR result shown in FIG. 17,the absorption dip of the flexible metamaterial has red shifted to ˜1.81μm. This red shift is mainly due to the refractive index change of thesurrounding medium (refractive index of optical adhesive and PET isabout 1.44). In FIG. 18C and FIG. 18D, three-dimensional rigorouscoupled wave analysis (RCWA) method is employed to calculate thereflection and transmission spectra on the absorber metamaterial, andexperimentally confirmed parameters of materials of gold, ITO, Cr, SiO2,and PET were used. Resonant absorption at wavelength of ˜1.81 μm canalso be observed in theoretical simulations. However, there are tworesonant dips around 1.2 μm in the measured reflection spectrum. In theRCWA calculation (FIG. 18C), the double dips are reproduced and ascribedto two localized resonant modes, as they are not very sensitive toincident angles. For the angle dependent calculation, TE polarized lightis used (electric field is perpendicular to incident plane) to fit theexperimental result. While the incident angle is changed from 0 to 45degree, reflection efficiency shows an increasing trend as light cannotbe efficiently localized under large angle incidence. However, theback-reflection efficiency in experiment (FIG. 18A) decreases obviously.This is because the current experimental setup (discussed in nextsection) only allows the collection of the back-reflection signal(incident and collection direction are same as each other), and thecollection efficiency is very low for large incident angles. In FIG.18B, transmission spectrum of the flexible metamaterial was measuredusing the same FTIR setup, the main difference is light was incidentfrom the air/PMMA interface. A Fano-type transmission peak is observedat wavelength ˜1.85 μm. At resonant wavelength, the transmissionefficiency from experiment is higher than that in the theoreticalsimulation (FIG. 18D). This could be due to defects on gold planar filmand the two-dimensional disc arrays, which enhances the efficiency ofleakage radiation and thus contribute to the higher transmissionefficiency in the measured results.

As shown in FIG. 19, bending PET substrate allows the measurement of theoptical response of absorber metamaterial under different curving shape.The shape of the bent PET substrate is controlled by adjusting thedistance between substrate ends (A and B). The angle for the resolvedback-reflection on the absorber device is measured by varying thebending conditions. From FIG. 19, the incident angle (90°−φ) isdetermined from the bending slope at the position of the metamaterialdevice. From FIG. 18A, it is observed that when the incident angleincreases from 0 to 45 degree, the intensity of the back reflectionbecomes weaker and the absorption dip becomes shallower. Nevertheless,it can be shown that the resonant absorption wavelength of the flexibleabsorber metamaterial is not sensitive to the incident angle of light.Devices made from the metamaterials can be made into highly sensitivesensors. This invention provides a novel technique in fabricatingmetamaterial devices on a flexible substrate. The flexibility allows thedevice to bend and stretch, altering the device structure. Since theresonant frequency of each device is a function of the device structure,the resonant frequency can be tuned by the bending and stretching of thesubstrate. Hence, another embodiment of the present invention is ametamaterial that enables a physical means to change the structure ofthe material, which leads to a change in its resonant frequency, withoutthe need to change the material composition. As such, an embodiment ofthe present metamaterial is a flexible plasmonic or metamaterialnanostructure device used as an electromagnetic wave absorber.

According to the aforementioned embodiments of the present invention, ahighly flexible tri-layer absorber metamaterial device working at NIRwavelength can be realized. Using the FCT method, a tri-layer golddisc/ITO/gold absorber metamaterial is transferred from quartz substrateto a transparent PET substrate using optically clear adhesive (e.g. acommercially available product manufactured by 3M). Furthermore, thetri-layer absorber metamaterial is encapsulated by PMMA thin film andoptical adhesive layer to form a flexible device. A FTIR experimentshowed that the absorber metamaterial works well on both the quartzsubstrate and the highly flexible PET substrate. Angle insensitiveabsorbing effects and Fano-type transmission resonance can also beobserved on this flexible metamaterial.

Moreover, the solution-free FCT technique described in this inventioncan also be used to transfer other visible-NIR metal/dielectricmultilayer metamaterial onto flexible substrate. The flexiblemetamaterial working at visible-NIR regime has many advantages bymanipulating light in three-dimensional space, especially when themetamaterial architecture is designed on curved surfaces. In anotherembodiment of the present invention, the FCT technique of the presentinvention can be adopted to transfer a hardened thin film on to asofter, flexible substrate.

Experimental Details on Transferring Thin Film onto Flexible Substrate

A Method is adopted for transferring Al₂O₃ thin films from rigidsubstrate to PET substrate using weak adhesive metal interlayers. Thisapproach is based on the referenced U.S. Non-Provisional patentapplication Ser. No. 13/726,127 filed on Dec. 23, 2012 and U.S.Non-Provisional patent application Ser. No. 13/726,183 filed on Dec. 23,2012, both of which claim priority from U.S. Provisional PatentApplication No. 61/579,668 filed on Dec. 23, 2011. One embodiment of thepresent invention is to use transparent polyester tape, applyingmechanical stress to separate the Al₂O₃ thin films altogether from thesacrificial metal layer. Then, the Al₂O₃ thin films are transferred tothe PET substrate and the sacrificial metal layer is etched away byacid.

First, a thin (i.e. 30-100 nm-thick) chromium (Cr) film is depositedonto a fused silica substrate followed by a thin (i.e. 30-100 nm-thick)silver (Ag) film being deposited on top of Cr. Then another layer ofmetal such as Ti film (3-10 nm thick) is deposited and this is forannealing process. Then, a Al₂O₃ thin film (e.g. 100-500 nm) isdeposited onto the metal layers. Annealing is then performed under thetemperature range 300° C.-800° C. per the embodiment of low temperatureannealing process of the present invention as disclosed earlier herein.Flexible transparent polyester tape with optical transmission higherthan 95% is attached to the Al₂O₃ film and the hardened Al₂O₃ thin filmis mechanically peeled back. The fabrication structure is schematicallyillustrated in FIG. 20. Due to different surface energies, the adhesionbetween Cr and Ag is weak and therefore can be easily overcome byapplying stress. The applied stress composed of both pure opening stressmode and shear stress mode. These two modes ensure that there is a cleanseparation between Ag and Cr. Under the applied stress, the hardenedAl₂O₃ thin film would detach itself from the rigid substrate altogetherwith the sacrificial Ag layer and flexible transparent polyester tape asshown in FIG. 21. Finally, the sacrificial Ag layer is etched away byimmersing the assembly as depicted in FIG. 21 by acid such as dilutedHNO₃ (1:1). Since the tape and Al₂O₃ thin film are acid-resistant, theetchant solution would only etch away the sacrificial Ag layer faster.Al₂O₃ is fully transferred to PET substrate depicted in FIG. 22 after Agthin film is completely etched away.

Results

FIG. 23 shows the sample fabricated for transfer of Al₂O₃ thin film. Onthe fused silica substrate, Cr was first sputtered onto the substratewith a typical thickness of 50 nm at a sputtering yield at about 5nm/min. Then, 50 nm Ag was deposited on top of it by e-beam evaporation.Finally, Al₂O₃ of about 200 nm thick was deposited to the assembly bye-beam evaporation.

FIG. 24 shows the peel off of Al₂O₃ film from fused silica substrate andCr after applying mechanical peel with a transparent tape. Al₂O₃detaches from the rigid substrate completely and smoothly without anycracks and bubbles together with Ag film and tape. Al₂O₃ is successfullytransferred to the flexible PET substrate after etching away thesacrificial Ag layer in acid.

In yet another embodiment of the present invention, the presentinventors have discovered through their trials, experimentations andresearch that to accomplish the task of depositing a layer of higherhardness thin film (of sapphire) onto a weaker hardness substrate e.g.soda lime glass (SLG), quartz and (toughened) glass. This combination isbetter than bare sapphire substrate. In nature, the higher hardnessmaterials would have worse toughness so sapphire substrate is hard toscratch but it is brittle to break. Therefore, using the weaker hardnesssubstrate with higher hardness thin film coating is best combination.Relative weaker hardness substrates have small fragmentationpossibility, good mechanical performance, and lower cost. The functionof anti-scratch is to achieve by using the high hardness thin filmcoating.

In this invention, there is provided a method to deposit a high hardnessalumina thin film on quartz substrate. The thin film thickness is in therange of 100-1000 nm. With post-deposit treatment such as thermalannealing at 25° C.-375° C., wherein 25° C. is considered roomtemperature, this invention has achieved hardness of more than 14 GPawhich is harder than uncoated soda lime glass which has typical hardnessof 8-8.5 GPa. This technology is called “Sapphire thin film coatedsubstrate”. Therefore, in terms of hardness, the sapphire thin filmcoated substrate is comparable to that of pure sapphire screen, and itsweight is almost the same as that of glass/quartz substrate which isroughly 66.6% comparing to pure sapphire substrate since the density ofquartz is only 2.65 g/cm³ while sapphire is 3.98 g/cm³. Since one cancut the substrate to the desired size then deposit the sapphire thinfilm, the fabrication cost and time is significantly reduced comparingto pure sapphire substrate.

It was found that the alumina thin film coated on soda lime glass viasputtering and with thermal annealing at 25° C. for 0.5 hour is harderthan uncoated soda lime glass. The film hardness was improved to greaterthan 14 GPa. Therefore, the hardness of annealed alumina thin film onsoda lime glass substrate is greater than the uncoated soda lime glass.

Moreover, under the present invention, the annealing process of aluminathin film on other substrates is conducted at room temperature or 25° C.

Deposition Process

Deposition substrate e.g. soda lime glass, quartz, glass.

Substrate temperature during deposition: from room temperature or 25°C.-1000° C.

Thin film thickness: 100 nm-1000 nm.

Thermal annealing time: 30 minutes-2 hours.

The deposition of alumina thin film is using sputtering or e-beam.

The thickness of the film deposited on substrates is about 100 to 1000nm. The deposition rate is about 1 Å/s. The substrate during depositionis without external cooling or heating. The film thicknesses aremeasured by ellipsometry method.

After deposition of alumina thin film on substrates, they are annealedfrom 28° C. The time ranges from 30 minutes to 2 hours, keeping on theparticular thermal annealing temperature.

The deposition substrates are including soda lime glass.

The nanoindentation results of aluminum oxide film on Soda lime glass(SLG) substrate with different post annealing conditions are showed inFIG. 25.

Further Embodiments of the Present Invention

In a further embodiment of the present invention, a layer of dopedaluminum oxide (sapphire) thin film can be deposited on sapphire thinfilm coated substrates acting as a strengthen layer. FIG. 26 shows thestructure of the sample. The doping materials need to have aconsiderable different in atom's size compare to aluminum, such asChromium or Chromium oxide; Magnesium or Magnesium oxide. The distinctsize of two atoms form an interlocking mechanism in the film, as aresult, surface hardness of film can be promoted. This interlockingmechanism is similar to chemical strengthen glass which is usingPotassium to replace Sodium in glass. The transmittance and hardness ofthe samples can be manipulated by the thickness, doping ratio and dopingmaterials of the strengthen layer.

The unique doping of the aluminum oxide (sapphire) thin film can alsoserve as a unique identifier of the specific aluminum oxide (sapphire)thin film coating applied on a given substrate. Thus, another embodimentof the present invention provides for a means for manufacturers to tracktheir manufactured doped sapphire coating by identifying the ratio andtype of dopant used in the deposited sapphire thin film coating.

In one of the experiments described in the present invention, when theratio of strengthen layer is 1:3 (aluminum oxide:chromium oxide) andthickness is around 30 nm on top of 200 nm sapphire thin film coatedsubstrate with thermal annealing at 300° C., the present invention hasachieved 17 GPa hardness in nano-indentation measurement (FIG. 27) whichis equivalent to 7.2-7.5 Mohs scale.

In another of the experiments described, when the ratio of strengthenlayer is 1:1 (aluminum oxide:magnesium oxide) and thickness is around 30nm on top of 200 nm sapphire thin film coated substrate no annealing atroom temperature or 25° C., the present invention has achieved greaterthan 17 GPa hardness in nano-indentation measurement (FIG. 28) which isequivalent to more than 7.2-7.5 Mohs scale. FIG. 28 presented data ofstrengthen layer at ratio of 1:1 (aluminum oxide:magnesium oxide)deposited in room temperature or 25° C. on different substrates, namelysoda lime glass (SLG) and chemically strengthened aluminosilicate glass(ASS). These data are presented in Table 9.

TABLE 9 Nanoindentation measurement results for strengthen layer is 1:1(aluminum oxide:magnesium oxide) on SLG and ASS. Calibrated Peakhardness peak hardness* Sample (GPa) (GPa) Quartz (QZ) 15.79 ± 0.24 14.0Fused silica (FS) 10.21 ± 0.10 9.25 Strengthened  8.5 ± 0.44 7.79aluminosilicate glass (ASS) Soda lime glass (SLG)  6.53 ± 0.20 6.12Mixed oxide on ASS (RT) 17.13 ± 0.40 15.14 Mixed oxide on SLG (RT) 17.94± 1.20 15.83 (*The calibrated values were based on the hardness of fusedsilica (9.25 GPa) and quartz (14.0 GPa) respectively.)

In FIG. 29, transmission of samples with different ratio of strengthenlayer has been shown. When strengthen layer's ratio is 1:2 (aluminumoxide:chromium oxide), the transmittance is around 80% in visible lightrange.

In FIG. 30, transmission of samples with 1:1 (aluminum oxide:magnesiumoxide) ratio of strengthen layer deposited in room temperature or 25° C.over two different substrates, namely soda lime glass (SLG) andchemically strengthened aluminosilicate glass (ASS) have been shown.When strengthen layer's ratio is 1:1 (aluminum oxide:magnesium oxide),the transmittance is greater than 90% in visible light range (400 nm to700 nm). These data are presented in Table 10.

TABLE 10 Transmission results for strengthen SLG and ASS.hen layer is1:1 (aluminum oxide:magnesium oxide) Average transmittance, 400-700 nmSample (%) Bare SLG 90.90 Bare ASS 92.37 Mixed oxide film on SLG 90.17Mixed oxide film on ASS 91.01

The hardness value of as-deposited sapphire thin film by e-beam orsputtering deposition is around 12-13 GPa which is about 5.5-6.5. Afterthermal annealing process, the film hardness is significantly improved.However, the softening point of glass is about 500° C. which mean thatthe annealing temperature cannot be high enough for sapphire tocrystalline. On the other hand, strengthen glass such as Corning Gorillaglass has even lower annealing temperature to 400° C. due to thestrengthen layer. After adding the doped aluminum strengthen layer, thefilm hardness has improved to 7.2-7.5 Mohs with 300° C. annealingtemperature at specific doping ratio of strengthen layer. This method isgreat improvement of surface hardness and de-stress problem onstrengthen glass substrate by lower the annealing temperature.

The procedure of depositing doped aluminum oxide strengthen layer on asapphire thin film coated substrate by sputtering deposition are givenas follows:

-   -   1. The deposition of Sapphire thin film follows the same        procedure and experimental details of “Sapphire Thin Film Coated        Substrate” of U.S. Non-Provisional patent Ser. No. 14/642,742        filed on Mar. 9, 2015, which claims priority from U.S.        Provisional Patent Application No. 62/049,364 filed on Sep. 12,        2014.    -   2. The base vacuum of chamber is higher than 5×10⁻⁶ mbar and the        deposited vacuum keeps higher than 5×10⁻³ mbar when the        deposition takes place.    -   3. The substrates are attached onto the sample holder at a        distance from the sputtering source, for example 150 mm. The        sample holder is rotated at 10 RPM when the deposition takes        place.    -   4. Co-sputtering technique is used to deposit a doped aluminum        oxide layer on the sample. Two sputtering guns which are contain        two different targets materials are operating simultaneously        during coating. And the doping ratio is controlled by the        sputtering power. E-beam deposition with similar arrangement is        also possible.    -   5. The thickness of doped aluminum oxide layer is 10 nm to 100        nm. The deposition rate is about 1-20 nm/min which depend on the        type of target used, such as oxide and metal targets. The        substrate during deposition is at room temperature or 25° C. and        active temperature is not essential. The film thicknesses can be        measured by ellipsometry method or other appropriate methods        with similar or better accuracy.    -   6. After deposited a doped aluminum oxide layer on sapphire thin        film coated substrates, they are annealed in a furnace from        50° C. to 1300° C. The temperature raising gradient should be        gradual for example 5° C./min and the decline gradient should        also be gradual for example 1-5° C./min. The annealing time is        ranged from 30 minutes to 10 hours within the specified thermal        annealing temperature range. Multiple-steps annealing with        different temperatures within the aforementioned range can also        be used to enhance the hardness and also reduce the micro-crack        of thin film.

Other possible dopants used are beryllium, beryllium oxide, lithium,lithium oxide, sodium, sodium oxide, potassium, potassium oxide,calcium, calcium oxide, molybdenum, molybdenum oxide, tungsten andtungsten oxide. In fact, an embodiment of the present invention hasspinel (MgAl₂O₄) produced in the doped aluminum oxide (sapphire) thinfilm coating on a softer substrate at the ratio of aluminumoxide:magnesium oxide being 1:1. From data in FIG. 31, it is observedthat when the doped aluminum oxide (sapphire) thin film with mixed oxideof MgO (at the ratio of aluminum oxide:magnesium oxide being 1:1) isdeposited using a physical deposition process unto field silica (FS)substrate; and anneal at different temperatures, namely at roomtemperature (RT) or 25° C., at 200° C. (S 200 A), at 400° C. (S 400 A),at 600° C. (S 600 A), at 800° C. (S 800 A) and at 1000° C. (M 1000 A),different level/concentrations of spinel is detected using XRD.Obviously, the most prominent peak of spinel is detected at 1000° C. (M1000 A). Nonetheless, even at room temperature (RT) or 25° C. XRDsignals of spinel are detected and co-incidentally the doped sapphirethin film with MgO is at its hardest when there is no annealing, i.e. atroom temperature (RT) or 25° C. Furthermore, at 1000° C. (M 1000 A), XRDpeak of alumina is also detected and under all tested annealingtemperature conditions, other than 1000° C. (M 1000 A), XRD peakindicating MgO is also detected. The physical deposition process used iseither an e-beam deposition or sputtering, wherein the deposition iswithout external cooling or heating and the entire process is done atroom temperature or 25° C. Furthermore, from data presented in Table 11,it can be seen that the aluminum oxide (sapphire) thin film layer isacting as to provide adhesion for the MgO mixed oxide to bind to thesubstrate when deposited under room temperature or 25° C.

TABLE 11 Thin film of aluminum oxide (sapphire):MgO (mixed oxide) at 1:1on different substrates at different thickness. 1. Structure 2. SLG 3.ASS Substrate/Mixed Oxide MgO (200 nm) Peel Off Peel Off (Depend onlocation) Substrate/Al₂O₃(340 nm)/Mixed Oxide MgO OK OK (20 nm)Substrate/Al₂O₃(50 nm)/Mixed Oxide MgO OK OK (200 nm)

A Further Embodiment of the Present Invention

Sapphire thin film has a high hardness mechanical property that means itis very rigid. So, when it is deposited on soft or flexible substrates,the difference in mechanical property between the sapphire and thesubstrates can cause the film to peel when the film is too thick orcrack due to the stress between substrate and film. For example,sapphire film begins to peel off from PMMA or PET substrate when thefilm thickness exceeds 200 nm.

In addition, the refractive index difference of the two materials meansthat light transmission through the layer can get trapped between thetwo materials. Thus, in a further embodiment of the present inventionthere is presented a buffer layer to act as mechanical and opticalintermediate layer. Mechanically the buffer layer would have hardnessintermediate to those of the soft substrate and sapphire film such thatit can relieve the high stress induced by the large hardness differenceof the aforesaid two materials. With the optimum thickness range,thicker sapphire film can be grown. Thicker sapphire film is desirablebecause anti-scratching requires a critical thickness to preventpuncture or piercing of the film. Furthermore, the buffer layer canreduce the interfacial stress and therefore better adhesion of the thinfilm.

Further Invention

The embodiments of the present invention provide:

-   -   1. A buffer layer with thickness 10-100 nm is deposited on to a        soft substrate such as PMMA and PET.    -   2. The deposition method can be thermal deposition, sputtering        or e-beam and the substrate does not need to be heated, that is        the deposition is without external cooling or heating.    -   3. The buffer layer material should have a mechanical hardness        higher than the substrate and lower than that of a typical        sapphire film, typical value range is 1-5.5 Mohs scale.    -   4. The refractive index of the buffer layer material should be        higher than that of the substrate but lower than that of a        typical sapphire film, typical value range is 1.45-1.65.    -   5. Such buffer layer can also improve the adhesion of the        sapphire because it reduces the stress generated due to large        difference in hardness.    -   6. An example of such material is silicon dioxide and SiO₂.

Using SiO₂ as buffer layer sapphire layer thickness can grow up to 300nm on PMMA before film peeling is observed. For sapphire film withoutSiO₂, peeling is observed at thickness at 150 nm and above (‘peel-off’thickness is termed as critical thickness). Therefore, the buffer layerhas improved the mechanical stability of sapphire film such that thecritical thickness is increased by 100% and more.

The introduction of SiO₂ as buffer layer has improved the overalloptical transmission of the coated substrate by not less than 2% acrossthe optical range. The transmission enhancement is brought about by thematching of refractive index of the buffer layer such that light canpass through from the substrate to the sapphire film with less loss. Theenhancement is due to reduction of differences in refractive index valuebetween the two material layers e.g. substrate and buffer layer, andbuffer layer and sapphire film. The reduction in refractive indexincreases the Brewster angle which defines the amount of light can passthrough from one medium to another across the interface. The bigger theBrewster angle the more light can pass through the interface. Thus,introduction of buffer layer between the substrate and sapphire filmincreases the amount of light transmitting through. This is shown inFIG. 32.

Hardness of at least 5 GPa or higher is achieved with total thickness of200 nm and above (buffer layer and sapphire film) when measured using anano-indenter as shown in FIG. 33. There is considerable improvement inhardness over uncoated substrate. For example, PMMA hardness is 0.3 Gpaand the hardness achieved is at 5.5 Gpa; that means it is more than 10times increase in hardness. This confirms that hardness and opticaltransmission enhancement can be achieved through introduction of bufferlayer between the soft substrate and the sapphire film.

The current embodiment of the present invention can also be applied onsoft, flexible substrates such as polymers, plastics, paper and fabrics.

Modifications and variations such as would be apparent to a skilledaddressee are deemed to be within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention relates to a method to transfer a layer of harderthin film substrate onto a softer substrate, especially onto a flexiblesubstrate. In particular, the present invention provides a method totransfer a layer of sapphire thin film onto a softer flexible substratee.g. PET, polymers, plastics, paper and even to fabrics via a flip chipprocess. The combination of a layer of harder thin film sapphiresubstrate onto a softer substrate is better than pure sapphiresubstrate. In nature, the harder the materials, the more brittle theyare. Thus, sapphire substrate is hard to scratch but it is easy toshatter, and the vice versa is also often true wherein quartz substrateis easier to scratch but it is less fragile than sapphire substrate.Therefore, depositing a harder thin film substrate on a softer substrategives the best of both worlds. Softer, flexible substrates are lessbrittle, have good mechanical performance and often cost less. Thefunction of anti-scratch is to achieve by using the harder thin filmsubstrate.

If desired, the different functions discussed herein may be performed ina different order and/or concurrently with each other. Furthermore, ifdesired, one or more of the above-described functions may be optional ormay be combined.

Throughout this specification, unless the context requires otherwise,the word “comprise” or variations such as “comprises” or “comprising”,will be understood to imply the inclusion of a stated integer or groupof integers but not the exclusion of any other integer or group ofintegers. It is also noted that in this disclosure and particularly inthe claims and/or paragraphs, terms such as “comprises”, “comprised”,“comprising” and the like can have the meaning attributed to it in U.S.Patent law; e.g., they can mean “includes”, “included”, “including”, andthe like; and that terms such as “consisting essentially of” and“consists essentially of” have the meaning ascribed to them in U.S.Patent law, e.g., they allow for elements not explicitly recited, butexclude elements that are found in the prior art or that affect a basicor novel characteristic of the invention.

Furthermore, throughout the specification and claims, unless the contextrequires otherwise, the word “include” or variations such as “includes”or “including”, will be understood to imply the inclusion of a statedinteger or group of integers but not the exclusion of any other integeror group of integers.

Other definitions for selected terms used herein may be found within thedetailed description of the invention and apply throughout. Unlessotherwise defined, all other technical terms used herein have the samemeaning as commonly understood to one of ordinary skill in the art towhich the invention belongs.

While the foregoing invention has been described with respect to variousembodiments and examples, it is understood that other embodiments arewithin the scope of the present invention as expressed in the followingclaims and their equivalents. Moreover, the above specific examples areto be construed as merely illustrative, and not limitative of thereminder of the disclosure in any way whatsoever. Without furtherelaboration, it is believed that one skilled in the art can, based onthe description herein, utilize the present invention to its fullestextent. All publications recited herein are hereby incorporated byreference in their entirety.

Citation or identification of any reference in this section or any othersection of this document shall not be construed as an admission thatsuch reference is available as prior art for the present application.

What is claimed is:
 1. A method for coating sapphire on to a substratecomprising, an e-beam evaporation or sputtering deposition process atroom temperature or 25° C., wherein sapphire is deposited directly on toa substrate selected from quartz, fused silica, silicon, glass, ortoughened glass to form a sapphire coated substrate, wherein thesubstrate during deposition is without external cooling or heating; andan annealing process, wherein said sapphire coated substrate is annealedunder an annealing temperature ranging between approximately roomtemperature or 25° C. and 2040° C. for an effective duration of time. 2.The method according to claim 1, wherein said substrate comprises atleast one material with a Mohs value less than that of said sapphire. 3.The method according to claim 1, wherein said sapphire is deposited as asapphire thin film on to said substrate.
 4. The method according toclaim 1, wherein said sapphire is deposited as a doped sapphire thinfilm on to said substrate.
 5. The method according to claim 4, whereinthe doped sapphire thin film is doped with doping element comprising oneor more of chromium, chromium oxide, magnesium, magnesium oxide,beryllium, beryllium oxide, lithium, lithium oxide, sodium, sodiumoxide, potassium, potassium oxide, calcium, calcium oxide, molybdenum,molybdenum oxide, tungsten, and tungsten oxide.
 6. The method accordingto claim 5, wherein the ratio of sapphire:doping element is in the rangeof 1:x, wherein x ranges from 1 to
 3. 7. The method according to claim1, wherein a thickness of said substrate is of one or more orders ofmagnitude greater than a thickness of said sapphire thin film.
 8. Themethod according to claim 7, wherein the thickness of said sapphire thinfilm is about 1/1000 of the thickness of said substrate.
 9. The methodaccording to claim 7, wherein the thickness of said sapphire thin filmis between 10 nm and 1000 nm.
 10. The method according to claim 7,wherein the thickness of said sapphire thin film is between 10 nm and100 nm.
 11. The method according to claim 1, wherein said effectiveduration of time is no less than 30 minutes and no more than 10 hours.12. A method for protecting a surface of a substrate by coating saidsurface with sapphire using the method according to claim
 1. 13. Ascreen fabricated by using the method according to claim 1 for use indisplays.